The invention pertains to a flexure-ring for centering a concave lens in a bore of a housing, such as a barrel of a telescope, particularly a plurality of lenses to be coaxially stacked in tandem, such as in a multilens camera or other precision optical device.
In a multilens system, it is common practice to coaxially stack lenses in tandem in the bore of a housing. Precision centering of the lenses is sometimes simply accomplished with spacing rings and without centering rings by matching the Coefficient of Thermal Expansion (CTE) of the lens material with a low CTE of the housing material. This, along with tight machining tolerances for a close fit to a bore. controls the lens to be axially centered in the bore without undue stress on the lens during nominal thermal excursions. However, in some instances, a low CTE housing material may not be suited to other needs of the lens assembly, such as when temporal stability and axial focus is important during anticipated significant temperature excursions, such as during outer space missions. In such cases, a complex arrangement is required to solve the thermal expansion problems resulting from such large temperature excursions, such as using precision control of temperature for the lens assembly which has its own problems.
To remedy this, lenses can be mounted in a housing with high CTE if the lenses are centered by other means. For a convex lens 10 shown in FIG. 1, centering can be accomplished by using prior-art, tangent-contact rings 11 and 12 fabricated of the same material as the housing 13 with a precision fit to the bore in the housing. Such tangent-contact rings are able to center a convex lens with tight tolerances and without constraining the lens radially, but they cannot be used to center concave lenses because of the lack of outwardly curved surfaces on both sides of the lens for tangent contact of the rings with the lens.
For a concave lens 14 shown in FIG. 2, a precision circumferential-contact ring 15 that surrounds the lens can be used, but that requires not only spacing rings 16, 17 and a tight precision fit of the ring 15 to the bore of the housing 18, but also close CTE matching for the material of the ring 15 to the material of both the housing 18 and the lens 14. However, in some applications, such as in a space telescope, machining tolerances that are otherwise sufficiently tight are too loose for required centering tolerances of 0.0007xe2x80x3. In that case, some means must be provided for relieving compression of the lens due to differences in CTE of the lens material vis-xc3xa1-vis that of the housing 18 and the peripheral contact ring 15. An object of the present invention is to provide a flexure-ring for centering a lens while at the same time relieving compression due to differences in the CTE of the ring 15 and the housing 18.
Flexible plastic rings have not been used for precision centering of lenses, because such rings could not be machined clean with the precision required for tolerances of 0.0007xe2x80x3, but it is now possible to use plastic rings due to the advent of durable elastomeric material that can be machined with that precision, such as from stock SP-1 made by E.I. DuPont de Nemours and Co. of synthetic resinous plastic, i.e., a plastic based on polyamide resin sold under the trademark Vespel. The availability of that material has made the flexure-ring of the present invention possible. However, other plastics capable of being machined clean with the requisite precision may already be or will become available. Consequently, it is not intended that the present invention be limited to the use of Vespel for the flexure-ring.
In accordance with the present invention, a flexure-ring for centering a concave lens in the bore of a housing is machined from a sheet of precision machinable plastic material with an outer diameter that has a precision fit to the inner diameter of a bore in a housing for a lens system and an inner diameter that has a precision fit to the concave lens, where the desired precision is of the order of about 0.0007xe2x80x3. This ring is then further machined to a larger inner diameter, but leaving lens contacting stubs of equal circumferential contact extent in 3N locations spaced apart by equal angles, where N is an integer equal to or greater than 1, such as 2, 3 or greater, and additionally machining a segment off the outside of the ring behind each contacting stub down to a chord of equal length for each stub greater than about twice the lens contacting length of each stub and perpendicular to the radius of the ring passing through the center of respective stubs. This leaves a ring with 3N equally spaced lens contacting stubs, each stub connected in the ring by two identical flexures, one on each side thereof, hence the term flexure-ring. In that manner, precision flexures on both sides of each contacting stub allow the stubs to center the lens with the requisite precision under varying ambient temperature conditions while compensating for any mismatch in the CTE between the housing, lens and ring material.
The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings.